This invention relates generally to pressure responsive electrical switches and more particularly to such switches used to monitor pressure levels of a fluid medium having adverse chemical and/or moisture characteristics.
Polyimide films are commonly used as diaphragm materials in pressure responsive electrical switches as a flexible interface between the pressure medium and the mechanical switch mechanism due to their excellent mechanical properties and outstanding resistance to change over a wide range of temperatures. However, these mechanical properties can degrade substantially when exposed to certain chemicals and/or moisture. Polyimide films exposed to water or water vapor undergo hydrolysis, which can adversely effect the film's ductility.
One supplier's literature describes a test where polyimide film (Kapton type H) is subjected to boiling water for 166 days prior to tensile testing. The film was said to retain 65% of its original tensile strength and 20% of its original elongation. This sharp decrease in elongation renders the film less ductile, i.e., embrittled. Similar tensile test results were found when immersing polyimide in moisture laden brake fluid at elevated temperature. For pressure switch diaphragm applications, this loss in ductility is highly undesirable and can lead to premature diaphragm failure due to film fracturing.
To alleviate this problem, the industry uses polyimide films which have been laminated with a protective overcoat, such as fluorinated ethylene propylene (FEP) resin. This composite film combines the high chemical inertness and water phobicity of the FEP fluorinated resin with the generally superior mechanical properties of polyimide. This overcoat effectively shields the polyimide from direct exposure to these adverse conditions, enhancing its overall performance.
However, pressure switches using diaphragms comprising FEP coated polyimide films, when used with certain fluid media, such as automotive brake fluids, are nevertheless subject to a primary mode of failure in which the pressure switch diaphragm ruptures resulting in brake fluid leakage. Initial film damage typically comprise cracks in the FEP protective layer followed by delamination of the FEP from the polyimide propagating from the cracked areas. Examination of test devices shows that fracturing of the FEP film precedes fracturing of the polyimide base film. Once this FEP overcoat fractures, the polyimide is exposed directly to the system fluid. This can cause the unprotected polyimide film to fail prematurely if the system fluid is detrimental to the base polyimide film, especially in dynamic applications requiring a high degree of film flexation, such as a pressure switch diaphragm.